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The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as Ga, for gigaannum) and evidence suggests that life emerged prior to 3.7 Ga.[1][2][3] Although there is some evidence of life as early as 4.1 to 4.28 Ga, it remains controversial due to the possible non-biological formation of the purported fossils.[1][4][5][6]
The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor.[7] Only a very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every microbial species on Earth presents a huge challenge."[8][9] However, only 1.75–1.8 million have been named[10][11] and 1.8 million documented in a central database.[12] These currently living species represent less than one percent of all species that have ever lived on Earth.[13][14]
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The earliest evidence of life comes from biogenic carbon signatures[2][3] and stromatolite fossils[15] discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.[16][5] In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago.[17][18]
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean eon and many of the major steps in early evolution are thought to have taken place in this environment.[19] The evolution of photosynthesis by cyanobacteria, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the ocean and then the atmosphere after depleting all available reductant substances on the Earth's surface, leading to the Great Oxygenation Event, beginning around 2.4 Ga.[20] The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga,[21][22] likely due to symbiogenesis between anaerobic archaea and aerobic proteobacteria in co-adaptation against the new oxidative stress. While eukaryotes may have been present earlier, their diversification accelerated when aerobic cellular respiration by the endosymbiont mitochondria provided a more abundant source of biological energy. Later, around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various algae that eventually overtook cyanobacteria as the dominant primary producers.
At around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions.[23] Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants).[24] However the origin and evolution of sexual reproduction remain a puzzle for biologists, though it did evolve from a common ancestor that was a single celled eukaryotic species.[25] Bilateria, animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago).[26]
The evolution of plants from freshwater green algae dated back even to about 1 billion years ago,[27] although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 Ga.[28] Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event.[29] (The long causal chain implied seems to involve (1) the success of early tree archaeopteris drew down CO2 levels, leading to global cooling and lowered sea levels, (2) roots of archeopteris fostered soil development which increased rock weathering, and the subsequent nutrient run-off may have triggered algal blooms resulting in anoxic events which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.)
Ediacara biota appeared during the Ediacaran period,[30] while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion.[31] During the Permian period, synapsids, including the ancestors of mammals, dominated the land,[32] but most of this group became extinct in the Permian–Triassic extinction event 252 Ma.[33] During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates;[34] one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods.[35] After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs,[36] mammals increased rapidly in size and diversity.[37] Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.[38]
Earliest history of Earth
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The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time.[43] The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing crust material into the orbit that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having a much stronger gravity than the early Moon, attracted almost all the iron particles in the area.[44]
Until 2001, the oldest rocks found on Earth were about 3.8 billion years old,[45][43] leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the Hadean.[46] However, analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life.[47][48][49]
Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar System, and the Earth should have experienced an even heavier bombardment due to its stronger gravity.[46][50] While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment.[51] This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although outgassing from volcanoes on Earth would have supplied at least half.[52] However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.[53]
Earliest evidence for life on Earth
The earliest identified organisms were minute and relatively featureless, and their fossils looked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga.[54] Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria,[55] with geochemical evidence also seeming to show the presence of life 3.8 Ga.[56] However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.[57][58] While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life,[54][59] although these statements have not been thoroughly examined by critics.
Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada,[17] although the evidence is disputed as inconclusive.[60]
Origins of life on Earth
Some biologists reason that all living organisms on Earth must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms.[62][63]
According to a different scenario[64][65][66] a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life[67] led to diversification through the development of a multiphenotypical population of pre-cells from which the precursor cells (protocells) of the three domains of life[68] emerged. Thus, the formation of cells was a successive process. See § Metabolism first: Pre-cells, successive cellularisation, below.
Independent emergence on Earth
Life on Earth is based on carbon and water. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide.[49] There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon, the element directly below carbon on the periodic table, does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because silicon dioxide is a hard and abrasive solid in contrast to carbon dioxide at temperatures associated with living things, it would be more difficult for organisms to extract. The elements boron and phosphorus have more complex chemistries but suffer from other limitations relative to carbon. Water is an excellent solvent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia, are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages.[69] Organisms based on alternative biochemistry may, however, be possible on other planets.[70]
Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.[71] Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other.[72][73]
Replication first: RNA world
Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA.[74] These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that offspring were likely to have different genomes from their parents, and evolution occurred at the level of genes rather than organisms.[75] RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding the capabilities of individual organisms.[75][76][77] Ribozymes remain as the main components of ribosomes, the "protein factories" in modern cells.[78] Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga.[79]
Although short self-replicating RNA molecules have been artificially produced in laboratories,[80] doubts have been raised about whether natural non-biological synthesis of RNA is possible.[81] The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA.[82][83]
In 2003, it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores.[84]
Membranes first: Lipid world
It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.[85] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves.[49] Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than outside.[86]
The clay hypothesis
RNA is complex and there are doubts about whether it can be produced non-biologically in the wild.[81] Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules.[87] Although this idea has not become the scientific consensus, it still has active supporters.[88]
Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles" and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes.[89]
A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.[90]
Metabolism first: Iron–sulfur world
A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts. Most of the steps required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents.[67]
Metabolism first: Pre–cells (successive cellularisation)
In this scenario, the biochemical evolution of life[67] led to diversification through the development of a multiphenotypical population of pre-cells,[64][65][66] i.e. evolving entities of primordial life with different characteristics and wide-spread horizontal gene transfer.
From this pre-cell population the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life[68] arose successively, leading first to the domain Bacteria, then to the domain Archea and finally to the domain Eucarya.
For the development of cells (cellularisation), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid cell walls by the invention of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere.[66]
This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. A scheme of the pre-cell scenario is shown in the adjacent figure,[66] where important evolutionary improvements are indicated by numbers.
Prebiotic environments
Geothermal springs
Wet-dry cycles at geothermal springs are shown to solve the problem of hydrolysis and promote the polymerization and vesicle encapsulation of biopolymers.[91][92] The temperatures of geothermal springs are suitable for biomolecules.[93] Silica minerals and metal sulfides in these environments have photocatalytic properties to catalyze biomolecules. Solar UV exposure also promotes the synthesis of biomolecules like RNA nucleotides.[94][95] An analysis of hydrothermal veins at a 3.5 Gya geothermal spring setting were found to have elements required for the origin of life, which are potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen.[96] Mulkidjanian and colleagues find that such environments have identical ionic concentrations to the cytoplasm of modern cells.[94] Fatty acids in acidic or slightly alkaline geothermal springs assemble into vesicles after wet-dry cycles as there is a lower concentration of ionic solutes at geothermal springs since they are freshwater environments, in contrast to seawater which has a higher concentration of ionic solutes.[97] For organic compounds to be present at geothermal springs, they would have likely been transported by carbonaceous meteors. The molecules that fell from the meteors were then accumulated in geothermal springs. Geothermal springs can accumulate aqueous phosphate in the form of phopshoric acid. Based on lab-run models, these concentrations of phoshate are insufficient to facilitate biosynthesis.[98] As for the evolutionary implications, freshwater heterotrophic cells that depended upon synthesized organic compounds later evolved photosynthesis because of the continuous exposure to sunlight as well as their cell walls with ion pumps to maintain their intracellular metabolism after they entered the oceans.[92]
Deep sea hydrothermal vents
Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds.[99] Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long.[100] Long chain fatty acids can be synthesized via Fischer-Tropsch synthesis.[101] Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in the assembly of vesicles.[102] Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients.[103] In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients.[104] Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts.[101] RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been a compartment for abiogenic processes.[105][106][107] A genomic analysis supports this hypothesis as they found 355 genes that likely traced to LUCA upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as gluconeogenesis, reverse incomplete Krebs cycle, glycolysis, and the pentose phosphate pathway, including biochemical reactions such as reductive amination and transamination.[108][109][101][110]
Life "seeded" from elsewhere
The Panspermia hypothesis does not explain how life arose originally, but simply examines the possibility of its coming from somewhere other than Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher Anaximander in the sixth century BCE.[111] In the twentieth century it was proposed by the physical chemist Svante Arrhenius,[112] by the astronomers Fred Hoyle and Chandra Wickramasinghe,[113] and by molecular biologist Francis Crick and chemist Leslie Orgel.[114]
There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large meteor impact, in which case the most credible sources are Mars[115] and Venus;[116] by alien visitors, possibly as a result of accidental contamination by microorganisms that they brought with them;[114] and from outside the Solar System but by natural means.[112][115]
Experiments in low Earth orbit, such as EXOSTACK, have demonstrated that some microorganism spores can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years.[117][118] Meteorite ALH84001, which was once part of the Martian crust, shows evidence of carbonate-globules with texture and size indicative of terrestrial bacterial activity.[119] Scientists are divided over the likelihood of life arising independently on Mars,[120] or on other planets in our galaxy.[115]
Carbonate-rich lakes
One theory traces the origins of life to the abundant carbonate-rich lakes which would have dotted the early Earth. Phosphate would have been an essential cornerstone to the origin of life since it is a critical component of nucleotides, phospholipids, and adenosine triphosphate.[121] Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for calcium ions. In a process called 'apatite precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals.[121] When attempting to simulate prebiotic phosphorylation, scientists have only found success when using phosphorus levels far above modern day natural concentrations.[98]
This problem of low phosphate is solved in carbonate-rich environments. When in the presence of carbonate, calcium readily reacts to form calcium carbonate instead of apatite minerals.[122] With the free calcium ions removed from solution, phosphate ions are no longer precipitated from solution.[122] This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body.[98] After all of the calcium is sequestered into calcium carbonate (calcite), phosphate concentrations are able to increase to levels necessary for facilitating biomolecule creation.[123]
Though carbonate-rich lakes have alkaline chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth's early carbon dioxide rich atmosphere.[98] Rainwater rich in carbonic acid weathered the rock on the surface of the Earth at rates far greater than today.[124] With high phosphate influx, no phosphate precipitation, and no microbial usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today.[98] Modeled pH and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on the origin of life.[98]
Similar to the process predicted by geothermal hot spring hypotheses, changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools.[125] This drying of the solution promotes polymerization reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer.[98][125][126] When washed away by further precipitation and wave action, researchers concluded these newly formed biomolecules may have washed back into the lake - allowing the first prebiotic syntheses on Earth to occur.[98]
Environmental and evolutionary impact of microbial mats
Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms.[127] To some extent each mat forms its own food chain, as the by-products of each group of microorganisms generally serve as "food" for adjacent groups.[128]
Stromatolites are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.[127] There has been vigorous debate about the validity of alleged stromatolite fossils from before 3 Ga,[129] with critics arguing that they could have been formed by non-biological processes.[57] In 2006, another find of stromatolites was reported from the same part of Australia, in rocks dated to 3.5 Ga.[130]
In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there.[128] Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms;[131][132] oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The source of hydrogen atoms used by oxygenic photosynthesis is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis.[133] From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes.[134]
Oxygen became a significant component of Earth's atmosphere about 2.4 Ga.[135] Although eukaryotes may have been present much earlier,[136][137] the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built.[138] The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association.[19]
Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient, well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton, provide the basis of most marine food chains.[19]
Diversification of eukaryotes
Chromatin, nucleus, endomembrane system, and mitochondria
Eukaryotes may have been present long before the oxygenation of the atmosphere,[136] but most modern eukaryotes require oxygen, which is used by their mitochondria to fuel the production of ATP, the internal energy supply of all known cells.[138] In the 1970s, a vigorous debate concluded that eukaryotes emerged as a result of a sequence of endosymbiosis between prokaryotes. For example: a predatory microorganism invaded a large prokaryote, probably an archaean, but instead of killing its prey, the attacker took up residence and evolved into mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants; and so on. After each endosymbiosis, the partners eventually eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them.[139][140][141] Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolising endosymbionts, and became oxygen-consumers later.[142] On the other hand, mitochondria might have been part of eukaryotes' original equipment.[143]
There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate eukaryotes at 2.7 Ga;[137] however, an analysis in 2008 concluded that these chemicals infiltrated the rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes.[144] Fossils of the algae Grypania have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised[22]), indicating that eukaryotes with organelles had already evolved.[145] A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga.[146] The earliest known fossils of fungi date from 1.43 Ga.[147]
Plastids
Plastids, the superclass of organelles of which chloroplasts are the best-known exemplar, are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out oxygenic photosynthesis.[138] Three evolutionary lineages of photosynthetic plastids have since emerged: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in the glaucophytes.[148] Not long after this primary endosymbiosis of plastids, rhodoplasts and chloroplasts were passed down to other bikonts, establishing an eukaryotic assemblage of phytoplankton by the end of the Neoproterozoic Eon.
Sexual reproduction and multicellular organisms
Evolution of sexual reproduction
The defining characteristics of sexual reproduction in eukaryotes are meiosis and fertilization, resulting in genetic recombination, giving offspring 50% of their genes from each parent.[149] By contrast, in asexual reproduction there is no recombination, but occasional horizontal gene transfer. Bacteria also exchange DNA by bacterial conjugation, enabling the spread of resistance to antibiotics and other toxins, and the ability to utilize new metabolites.[150] However, conjugation is not a means of reproduction, and is not limited to members of the same species – there are cases where bacteria transfer DNA to plants and animals.[151]
On the other hand, bacterial transformation is clearly an adaptation for transfer of DNA between bacteria of the same species. This is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex.[152][153] This process occurs naturally in at least 67 prokaryotic species (in seven different phyla).[154] Sexual reproduction in eukaryotes may have evolved from bacterial transformation.[155]
The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect.[149] Nevertheless, the great majority of animals, plants, fungi and protists reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then.[156] How sexual reproduction evolved and survived is an unsolved puzzle.[157]
The Red Queen hypothesis suggests that sexual reproduction provides protection against parasites, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical clones than those of sexual species that present moving targets, and there is some experimental evidence for this. However, there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species.[149] Furthermore, contrary to the expectations of the Red Queen hypothesis, Kathryn A. Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.[159] In addition, biologist Matthew Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host.[160]
Alexey Kondrashov's deterministic mutation hypothesis (DMH) assumes that each organism has more than one harmful mutation and that the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the gene pool by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However, the evidence suggests that the DMH's assumptions are shaky because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of synergy between harmful mutations.[149]
The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This genetic drift is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction.[157] Other combinations of hypotheses that are inadequate on their own are also being examined.[149]
The adaptive function of sex remains a major unresolved issue in biology. The competing models to explain it were reviewed by John A. Birdsell and Christopher Wills.[161] The hypotheses discussed above all depend on the possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose and is maintained as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct.[155][162]
Multicellularityedit
The simplest definitions of "multicellular", for example "having multiple cells", could include colonial cyanobacteria like Nostoc. Even a technical definition such as "having the same genome but different types of cell" would still include some genera of the green algae Volvox, which have cells that specialize in reproduction.[163] Multicellularity evolved independently in organisms as diverse as sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime molds and myxobacteria.[22][164] For the sake of brevity, this article focuses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the evolution of biological complexity could be regarded as "rather anthropocentric".[23]
The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell,[166] increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis;[167] the ability to create an internal environment that gives protection against the external one;[23] and even the opportunity for a group of cells to behave "intelligently" by sharing information.[165] These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.[167]
Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity.[167]
The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eukaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function.[167]
By comparing the composition of transcription factor families and regulatory network motifs between unicellular organisms and multicellular organisms, scientists found there are many novel transcription factor families and three novel types of regulatory network motifs in multicellular organisms, and novel family transcription factors are preferentially wired into these novel network motifs which are essential for multicullular development. These results propose a plausible mechanism for the contribution of novel-family transcription factors and novel network motifs to the origin of multicellular organisms at transcriptional regulatory level.[168]
Fossil evidenceedit
The controversial Francevillian biota fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular, if they are indeed fossils.[40] They may have had differentiated cells.[169] Another early multicellular fossil, Qingshania, dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called Bangiomorpha, dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism.[167] The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells.[147] The "string of beads" organism Horodyskia, found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan;[22] however, it has also been interpreted as a colonial foraminiferan.[158]